1. Field of the Invention
This invention relates to an apparatus for delivering precise volumes of a liquid repeatedly. More particularly, this invention relates to an apparatus which utilizes a pressurized hydraulic liquid to deliver precise volumes of a sample liquid.
2. Description of Prior Art
A variety of industries require the delivery of precise volumes of a liquid to an area of use. For example, precise volumes of a photoresist composition are delivered to a silicon wafer substrate in order to form a photoresist layer of uniform thickness onto the wafer. The requirement for precise volume delivery must be effected by an apparatus which is capable of delivering the same volumes of samples repeatedly over long time periods.
A wide variety of apparatus presently are available for delivering liquids. Apparatus which utilize a bellows are undesirable since the walls of the bellows are compliant. The volume of fluid displaced thus will vary depending on the pressure of the fluid.
Apparatus which utilize a pressurized gas to move a mechanical element such as a diaphragm which, in turn, displaces a fluid also are undesirable since gases are compressible and the volume of the gas depends upon the pressure to which the gas is subjected. Thus, volume of fluid displaced will vary depending on the pressure of the fluid.
Apparatus which utilize a solenoid to move a piston or the like also are undesirable since normal variations in electrical power delivered to the solenoid results in a variance of piston stroke length and thus, a variance in volume of fluid delivered. It is also difficult to control a position of a solenoid arm to other than its two extreme positions.
Accordingly, it would be desirable to provide an apparatus capable of delivering precise volumes of liquid repeatedly. In addition, it would be desirable to provide such an apparatus which is capable of delivering precise volumes of liquid over extended time periods.